Antenna



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United States Patent ANTENNA Edwin S. Lewis, Merchantville, N.J., assignor to Radio Corporation of America, a corporation of Delaware Filed May 26, 1958, Ser. No. 737,751

9 Claims. (Cl. 343-100) The present invention relates to antennas and particularly to an improved feed system for a scanning antenna.

In many radar applications it is necessary to employ an antenna of relatively large size to obtain high gain during transmission and reception. An example is an antenna first used a number of years ago for the lpge- 1s antenna uses a so-called four horn feeda cluster of four horns located at or near focus of an electromagnetic lens. The energy transmitted from and received by the horns is mixed in such manner that sum and difference antenna patterns are produced during transmission and reception respectively. This antenna has come to be known as a monopulse antenna.

The characteristics of the primary beam pattern and the lens diameter of the antenna above were such that the lens had to be designed to have a relatively long focal length. The lens was on the order of 12 feet, the primary beam pattern was about 68, and the focal length therefore was roughly 9 feet. The F/D (focal length to lens diameter) ratio was roughly 0.75. However, since the lens was mounted on one side of the axis about which the antenna rotates and the four horn feed on the other, the problem of maintaining the horn and lens aligned during rotation was not serious. If, on the other hand, it is attempted to use the four horn feed with a parabolic dish, serious mechanical difliculties are introduced. Now, the four horn feed is spaced from the axis about which the antenna rotates a distance at least equal to the antenna focal length. The long focal length makes it diflicult to prevent the feed from becoming misaligned with respect to the reflector when the antenna is slewed. Any such misalignment produces serious tracking errors.

An improved four horn feed system was subsequently developed in which the in-phase radiation from the horns, in one plane, was in two different modes, one an odd harmonic of the other. For reasons not necessary to discuss in detail here, this substantially broadened the feed horn pattern and permitted a reduction of the F/D ratio to approximately 0.3, and a reduction in the focal length to slightly more than 4 feet (for a 12 foot lens or reflector aperture). This made it practical to use a four horn feed with a parabolic dishan advantageous structure in view of the lower losses inherent in the dish (reflective) type of structure compared to those of the lens (refractive) type of structure. Side lobes were also reduced substantially in view of the broader feed horn pattern. However, this antenna and the previous one described were suitable only for the radiation of linearly polarized energy.

In some radar applications, it is desirable to be able to radiate circularly polarized energy. This facilitates tracking objects outside the atmosphere since rotation of polarization can be expected there. It also affords a means of discrimination against decoys in connection with defense against intercontinental ballistic missiles, for example. If a conventional, circularly polarizd mono pulse four horn feed of the type described above were used for this purpose, the four horn feed would again require that a long focal length reflector be used. Each horn would have to be capable of supporting the TE and TE modes. This would require that each horn be greater than a half wavelength on each side and that the overall aperture of the four horn cluster be greater than a wavelength on each side. Since the width of the primary beam pattern varies inversely as the size of the horn aperture, this places an upper limit on the width of the beam pattern and a limit on any further reduction in reflector focal length.

An object of the present invention is to provide an improved feed system which has a broad primary beam pattern and accordingly which can be used with a relatively short focal length reflector.

It is another object of the present invention to provide an improved antenna of the above type which is suitable for monopulse operation.

It is another object of the invention to provide an improved antenna of the above type which can produce sum and difference patterns and which can radiate and receive linearly, elliptically or circularly polarized waves.

Another object of the invention is to provide an improved antenna of the immediately preceding type which has a radiating aperture substantially smaller than that which would be required for the more conventional four horn feed. This is advantageous in two respects. One is that it permits a broader primary beam pattern. The other is that it creates a smaller shadow producing obstacle in the secondary beam pattern.

The radiator or receiver of the present invention comprises a radio-frequency horn of sufficiently large crosssection to support the propagation of energy in a plurality of modes including the TB TE TE and TM modes. The horn has an aperture through which energy is radiated or received and a closed end. A plurality of transmission line connections are located near the closed end of the horn for applying energy to or receiving energy from the horn in the different modes the horn is capable of propagating.

In a preferred form of the invention, the horn is of square cross-section. To be capable of supporting the TE and TM modes, the diagonal dimension of the horn aperture need only be slightly more than a wavelength at the operating frequency. An aperture of this size is more than sutficient to support the TE and TE modes. Thus the aperture is only about 0.7 the size that a comparable four horn feed aperture would be. The smaller aperture means a correspondingly broader primary feed pattern with the result that a much shorter focal length reflector can be employed.

The transmission line connections to the horn may comprise connections for rectangular waveguides, the long dimensions of which are perpendicular to the horn axis and the short dimensions of which are parallel to the horn axis. In this case each wall of the square horn includes one waveguide connection near the closed end of the horn. Other types of transmission line connections such as coaxial line connections may, however, be used instead of waveguide connections.

The type of radiation or reception produced by the horn depends upon the phase of the energy at the transmission line connections. For example, if it is desired to radiate a circularly polarized wave, energy is applied to successive ones of the four connections at relative phases of 0, and 270. If it is desired to radiate a linearly polarized wave, energy may be applied to two opposite ones of the connections in opposite phase. Many other ways of operating the feed are possible.

The invention will be described in greater detail by reference to the following description taken in connection with the accompanying drawings in which:

Fig. l is a partially broken away, perspective view of a feed system according to the present invention;

Fig. 2 is a diagram to illustrate several of the modes the square horn of Fig. l is capable of propagating;

Fig. 3 is a cross-section through the horn showing the manner in which the TB mode may be set up in the horn;

Figs. 4a and 4b are schematic views representing the horn in cross-section to explain the modes set up in the horn when the antenna is on target and when the antenna is slightly above the target;

Fig. 5 is a drawing showing three modes which may be simultaneously excited in the horn;

Fig. 6 is a cross-section through the horn showing a wave configuration which excites two opposite waveguides in phase;

Fig. 7 is a schematic drawing showing how different modes excited in the horn can be separated in an external circuit; a

Fig. 8 is a schematic drawing showing the mode configuration which may be set up in the horn when the antenna is off target in azimuth;

Fig. 9 is a perspective view of an antenna including the parabolic reflector and the horn of this invention;

Figs. IOa-lOd are radiation patterns of the antenna of Fig. 9 under varying conditions; and

Fig. 11 is a block circuit diagram of the part of a radar system in which the horn of the present invention may be used.

Throughout the figures, similar reference numerals are applied to similar elements.

The horn shown in Fig. l is of square cross-section and includes an aperture 12 from which energy is radiated and received and a closed end 14. Four rectangular waveguides 16, 18, and 22 open into the four walls of the horn, respectively near the closed end 14 of the horn. The rectangular waveguides are arranged with their short rectangular dimensions parallel to the horn axis and their long rectangular dimensions perpendicular to the axis. In a preferred form of the invention an impedance matching element 23 is located at the closed end of the horn equally spaced from the four walls thereof. In the embodiment illustrated, this element is conical in shape. Its base diameter is about half that of the base of the horn, and its height is slightly larger than the short dimension of the waveguide. Other shapes and dimensions are possible. For example, the matching means may be of pyramidal shape. As another example, the sides of the pyramid or cross-section of the cone may be curved rather than straight.

Fig. 2 shows the electric field configurations of the various modes in which the horn can propagate electromagnetic energy. The diagonal dimension of the horn must be slightly greater than a wavelength for these modes to be propagated.

In its transmitting condition, energy is fed into the waveguide connections to the horn. The relative phases and amplitudes of this energy determine the polarization of the radiation from the horn. The energy radiated from the horn is normally, but not necessarily, fed into a parabolic reflector and focused by the reflector into a narrow beam. A typical antenna may be like the one shown in Fig. 9. The reflector consists of a parabolic dish 24. The feed system including the square horn 10 is located with its aperture at or near the focal point of the reflector. The supporting structure for the horn 10 may consist of the waveguides 16, 18, 20 and 22, as shown in the figure.

It is to be understood that Fig. 9 is meant to be illustrative of the invention rather than limiting. For example,- the reflector can be other than parabolic and, in fact, in some circumstances the primary beam pattern itself may be employed without a reflector. The transmission line supporting arrangement too may be different than what is shown. In one practical system, for example, the feed system included coaxial lines which were coupled to short lengths of waveguides connected to the horn. Such lines are more practical than waveguides at lower frequencies. Another alternative is to connect the coaxial lines directly to the square horn, eliminating the waveguides completely. A number of known coaxial line-to-waveguide coupling arrangements may be used to connect to the horn.

As mentioned above, the mode propagated in the horn 10 depends upon the phase of the energy exciting the waveguides. If it is desired to radiate a linearly polarized wave, the electric vector of which is perpendicular to the earth, waveguides 18 and 22 are excited 180 outof phase. This is illustrated in Fig. 3. Note that the two out-of-phase components reinforce in the born 10. The out-of-p-hase condition of the energy in waveguides 18 and 22 is represented by the arrows which face opposite directions.

Returning for a moment to Fig. 1, it can readily be shown that if waveguides 16 and 20 are excited 180 out-of-phase, the TE mode will be set up in the horn.

If other than vertical and horizontal polarization is required, energy may be fed into opposite pairs of waveguides simultaneously. By proper choices of relative phases and amplitudes of the excitation of the waveguides, any arbitrary polarization can be obtained. For example, if the TB and TE modes are excited in equal amplitude and out-of-phase, circularly polarized radiation results. In other words, if energy is applied to waveguide 16 in a reference phase and energy is then applied to waveguides 18, 20 and 22 in 90, and 270 phase respectively, circularly polarized radiation will result. On the other hand, if energy is applied to Waveguides 16, 18, 20 and 22 at phases of 0, 0, 180, 180 respectively, linear polarization will result at an angle of 45 to a horn wall. Any intermediate condition can be obtained by proper adjustment of the phases and amplitudes. Circuits for obtaining the variable excitation are conventional and need not be described here.

The manner in which the horn receives energy is best understood by considering the born at the focal point of the parabolic dish or other focusing means as, for example, is shown in Fig. 9. Energy incident on the dish parallel to its axis is focused in a diffraction pattern at the focal point. This may be considered as an area of energy incident on the horn and may be represented by the circle on the square horn of Fig. 4a. The arrows represent vertical polarization of the incident wave. Since this incident energy is symmetrically located on the horn (the antenna is on target), only the TE mode is excited. It is coupled out of the born with waveguides 18 and 22 which are excited out-of-phase. The energy from the two waveguides can be combined in a magic T or other hybrid junction, as will be explained in greater detail later.

If the energy incident on the parabolic reflector is coming from slightly below the axis of the reflector, the area of the energy focused on the horn is moved to the upper part of the horn as shown in Fig. 413. Since the excitation is not symmetrical, the fundamental and other modes will be set up in the horn.

The electric field configuration of three different modes which are excited in the horn when the energy incident on the horn is non-symmetrical is shown in Fig. 5. The three modes are the TE TM and the T13 modes. It can be seen from the figure that the three modes reinforce toward the top of the horn and cancel toward the bottom of the horn, thus approximat ing the asymmetrical excitation shown in Fig. 4B. The TE mode energy: is extracted from the horn in the manner'shown in Fig. 3-.- Thus, as' previously-explained;

this mode excites the two waveguides 18, 22 180 outof-phase. The TM and TE modes combine and excite waveguides 18 and 22 in the manner shown in Fig. 6. As can be seen, these modes excite the two waveguides in phase.

The TE component can be separated from the TM and TE components in the manner shown in Fig. 7. The horn and two waveguides 18 and 22 are shown schematically. Wave guides 18 and 22 are connected to the symmetrical arms E and E of a magic T hybrid junction 24. As is well known, when the input to arms E and E is 180 out-of-phase, all of the energy goes to the E-plane arm E of the magic T. Thus, all of the energy in the TE mode goes to arm E On the other hand, when the energy fed to the symmetrical arms E and E is in phase, it is coupled to the H-plane arm H of the magic T. Thus, all of the energy in the TM and TE modes goes to the H arm. The signals extracted from the H and E arms of the magic T form the reference and elevation error signals, respectively. When the antenna is on target, the error signal is zero (see Fig. 100) and the reference signal is maximum (see Fig. 10a). These signals may be processed and applied to the antenna driving means via a servo system for repositioning the antenna in a sense to reduce the error signal to zero. A brief description of a system incorporating such repositioning means is given later.

The horizontal components of the TM and TE modes excite waveguides 16 and 20 (Fig. 1) but these components tend to cancel, as can be seen in Fig. 5, and therefore the excitation is small.

If energy is incident on the parabolic reflector from off axis in the azimuth plane, the horn is excited asymmetrically by the vertically polarized signals as shown in Fig. 8. This excitation does not produce either the TM or TE modes and thus, no error signal is produced. However, horizontally polarized components of the received signals excite the horn in exactly the same manner as shown in Figs. 4b, 5 and 6 but with the whole system rotated 90. This could be illustrated by a drawing similar to the one shown in Fig. 6, however, the waveguides shown would be waveguides 16 and 20 rather than 18 and 22. The signals received by waveguides 16 and 22 would be applied to a circuit similar to the one shown in Fig. 7 to obtain the azimuth reference and azimuth error signals. These would be used for repositioning the antenna in azimuth to reduce the azimuth error to zero. 1

-A practical antenna such as described above may have the following parameters: dish diameter=42 inches; frequency=5650 megacycles; horn dimensions=1.872 inches square and 4 inches long; focal length=14.5 inches.

Figs. 1012-10d show receiving patterns for the antenna above. Fig. 10a shows the amplitude of the TE mode as a function of the angle, off axis of the parabolic reflector in the plane of the electric field (E-plane) and Fig. 10b shows the amplitude of the TE mode as a function of the angle 06 axis of the parabolic reflector in the plane perpendicular to the electric field or the H-plane. Note that in each case the beam is extremely sharp. The side lobes are somewhat more pronounced in the H-plane than in the E-plane but, in both cases, they are well down from the maximum beam power and are relatively widely spaced from the center of the beam.

The patterns of Figs. 10a and 10b are not exactly the same, as a square horn operating in the TE mode has diflerent radiation patterns in the E and H-planes. The horn described above, for example, produced a pattern having a width at the db down points of 121 degrees in the E-plane and 166 degrees in the H-plane. The F/D ratio used was 0.345 which gives an included angle from the focal point to the edge of the dish of 140 degrees. The dish used was circularly symmetrical. The

edge of the dish was thus under illuminated in the E-plane, accounting for the wider beam and lower side lobes, and over illuminated in the H-plane accounting for the narrower beam and higher side lobes.

Figs. 10c and 10d show the amplitude of the combined TM and TE modes as a function of the angle off axis of the parabolic reflector in the plane of the electric field. This represents the in phase excitation of waveguides 18 and 22 as illustrated in Fig. 6. The crosstalk pattern of Fig. 10d represents the excitation of waveguides 16 and 20 by the arrival of signals from off axis in the elevation plane. This represents an azimuth error signal produced by an elevation error. However, since the monopulse system is a null-seeking system, this does not produce any serious errors.

A portion of a radar system in which the present invention may be employed is shown in block form in Fig. 11. The transmitter 30 produces high-power radiofrequency pulses and applies them through hybrid junction 32 to duplexers 34 and 36, respectively. The hybrid junction may be a magic T or the like in which case the fourth arm 38 is preferably terminated in a matched load 40. The duplexers 34 and 36 are conventional and may include transmit-receive switches. A function of the duplexer is to couple the transmitter energy to the hybrid junction leading to the horn and to prevent the transmitter energy from reaching the receiver.

The energy from duplexer 34 is applied to a magic T 42 (or other type of hybrid junction) so that one-half the incident power appears at each of arms 44 and 46. In a similar manner the signal from duplexer 36 is applied to magic T 48 and one-half the incident power appears at arms 50 and 52 respectively. Thus, the arms 44, 46, 50 and 52 each carry the same amount of power.

The embodiment of the invention chosen for purposes of illustration is one which transmits a circularly polarized wave. Accordingly, the lengths of the transmission lines (coaxial lines, waveguides or the like) leading from the magic Ts 42 and 48 to the horn 10 are such that the energy is applied to the four horns at phases of 0, and 270, respectively. These are noted on the drawing. Note, in this connection, that energy is applied to each magic T via the E-plane arm of the T so that the output energy of the magic T at the symmetrical arms is 180 out-of-phase. Accordingly, the lines leading from arms 44 and 46, for example, to the 90 and 270 input connections to the horn should be of equal length. The same holds for the lines leading from the symmetrical arms 50 and 52 of magic T 48 to the 0 and 180 input connections to the horn 10.

The energy radiated from horn 10 is directed at parabolic dish 54 and reflected into space in a narrow, pencil beam. The beam may be of any width depending upon the reflector diameter. Typically, it is on the order of 3 to 5 at the three db down points, as shown in Figs. 10a and 10b. A portion of the energy striking reflecting targets returns to the dish and is picked up by horn 10. The target echoes excite the horn in the manner described previously. Considering first the signals derived from the horizontally polarized energy received, the azimuth reference signal passes from the horn 10 to the magic T 42. Remember that the azimuth reference signal consists of 180 out-of-phase components picked up by the 90 and 270 arms of the horn so that it passes from arms 44 and 46 of the magic T to arm 56 thereof. This energy goes through duplexer 34 to the azimuth reference receiving system 58.

The azimuth error signal consists of the TM and TE components so that the energy applied to arms 44 and 46 of the magic T is in phase. This energy passes through the magic T to the H arm thereof 60. The error signal goes from arm 60 through a transmitreceive tube 62, whose purpose is to protect the azimuth error receiver during the transmitter pulse, to the azimuth error receiver 64.

The output of the azimuth reference receiver 58 is divided into two parts, one of which is applied to the azimuth error detector 66. The second input to the azimuth error detector is the output of the azimuth error receiver 64. The azimuth error detector is basically a discriminator or a product detector. The signal output to the servo system 68 is the product of the two input signals to the detector times the cosine of the phase angle between the two signals. Thus, the servo signal goes to zero when the error signal is zero or, in the more practical case, when the error signal is in a phase quadrature with reference signal. The signal produced by the detector 66 drives the azimuth error servo system 68, which in turn, produces a signal which actuates the azimuth drive means 70. The latter repositions the antenna in the azimuth direction until the azimuth error is reduced to zero.

Elevation reference and error signals derived from the vertically polarized energy received follows paths similar to those described for the azimuth portion of the system. The circuits are shown in the right portion of the drawing leading from magic T 48. When there is an elevation error, the system produces an elevation error signal which eventually causes the elevation drive means to reposition the antenna in the elevation plane in a sense to reduce the elevation error signal to zero.

The portions of the azimuth and elevation reference signals which are not applied to the error detectors are cross-coupled through 90 phase shifters 72 and 74 and recombined in two detectors 76- and 78. When coupled in this manner, one of the detectors is sensitive only to a left hand circular polarization and the other to only right hand circular polarization. The outputs of one or both of these detectors (depending on the target echoes characteristics) are applied to conventional radar range and display circuits.

What is claimed is:

1. A radiator or receiver comprising, a horn of square cross-section having an open end and a closed end, the cross-sectional dimension of the horn being sufficiently large to support the propagation of energy in a plurality of modes; four transmission line connections to the horn near the closed end thereof, one on each side of the horn; means for detecting out-of-phase energy transmitted from said horn to a pair of said connections at opposite sides of said horn; and means for detecting in-phase energy transmitted from said horn to said pair of connections.

2. A radiator or receiver as set forth in claim 1, wherein said transmission line connections comprise connections for rectangular waveguides the long dimensions of which are perpendicular to the horn axis and short dimensions of which are parallel to the horn axis.

3. A radiator or receiver comprising, a radio-frequency horn of square cross-section having an open end through which energy may be emitted or received and a closed end, the cross-sectional area of the horn being sufficient to support the propagation of energy in a plurality of modes; four waveguides connected to the horn near the closed end thereof, one to each side wall of the horn; and means for applying energy to the four waveguides at the same frequency with the energy to succeeding ones of the waveguides at relative phases of 90, 180 and 270.

4. In combination, a radio-frequency horn of sufficiently large cross-section to support the propagation of energy in TE and TE modes; a pair of transmission line connections to the horn to which energy may be applied for exciting the horn in the TE mode and from which energy may be taken when the horn is excited in the TE mode; a second pair of transmission line connections to the horn to which energy may be applied for exciting the horn in the TE mode and from which energy may be taken when the horn is excited in the TE mode, means for detecting 180 out-of-phase energy applied from said horn to each pair of said connections; and means for detecting in-phase energy applied from said horn to each pair of said connections.

5. In the combination as set forth in claim 4, said horn being of square cross-section throughout its length and having a diagonal dimension slightly greater than one wavelength at the operating frequency. A

6. In combination, a radio-frequency horn of sufl"1 ciently large cross-section to support the propagation of energy in the TE and TM modes, said horn having an aperture and a closed end; four transmission line connections to the horn symmetrically spaced around the horn periphery near the closed end thereof; and means coupled to said connections for separating the energy in said horn in the TE and T M modes from the energy in said horn in the TB and TE modes.

7. In the combination as set forth in claim 6, said horn being axially symmetrical in cross-section.

8. An antenna which is capable of radiating and receiving a circularly polarized wave comprising, in combination, a reflector having a focus; a horn having an aperture at said focus and which is capable of supporting the propagation of energy in the TB TE TE and TM modes; four transmission line connections to the horn equally spaced around the periphery of the horn for exciting a circularly polarized wave in the horn; and means coupled to said connections for separating energy received by the horn and propagating therein in the TE and TM modes from that received by the horn and propagating therein in the TE and TE modes.

9. In combination, a radio frequency horn of sufficiently large cross-section to support the propagation of energy in the TE TM TE and TE modes, said horn having a n aperture and a closed end; four transmission line connections to the horn symmetrically spaced around the horn periphery near the closed end thereof; and a pair of hybrid T junctions the opposite E-arms of one being connected to one opposite pair of said connections and the opposite E-arms of the other being connected to the other opposite pair of said connections.

References Cited in the file of this patent UNITED STATES PATENTS 2,534,271 Kienow Dec. 19, 1950 2,651,759 Lamont et a1 Sept. 8, 1953 2,686,901 Dicke Aug. 17, 1954 2,786,132 Rines Mar. 19, 1957 OTHER REFERENCES Le Vine et al.: Dual-Mode Horn Feed for Microwave Multiplexing, Electronics, September 1954, pp. 162-164. 

